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Appendix VI.                          1113

The law of magneto-electric induction, discovered by Faraday,
affirms that whenever a conductor is moved across such force
lines, an E.M.F. (electro-motive force) is set up in the conductor,
and a current flow is caused by such E.M.F. if the circuit of the
conductor be closed. Also the direction of the current will bear
relation to the direction of motion of the conductor, and the
intensity of the E.M.F. will depend upon the speed with which

I                    the conductor cuts the lines of force.

\                        Any m&gnet, permanent or electric, causes a state of electric

stress in the medium between its poles (Fig. 977), and the
intensity of the stress is conveniently measured as ' force lines per
sq. centimetre' on the cross-sectional area of the poles. If N =
total number of force lines, and At = ampere turns of the coils
of an electro-magnet (viz., Amperes sent through x Turns of the

Coil):                                  XT       .

7                                   N oc At

in any particular magnetic circuit.

Consider now the application of Faraday's law in a simple
manner, Fig. 978.. Two rods DDare connected by a galvano-
meter c and a loose rod <?, thus forming a closed circuit, the
whole being placed across the force lines existing between the
magnet poles A and B. If e be moved to right or left to cut these
lines, an E.M.F. is set up in the circuit, which is detected by the
deflection of the galvanometer needle. The direction of current
thus caused depends on the direction f motion of <?, and the
E.M.F. on its speed.

Next imagine a rotary-moving circuit, Fig. 979, consisting of a
stiff wire A connected at its open ends to the cylinders B B, and
forming &pair of conductors c^ and a%. The circuit is actually
completed by brushes from slip rings connected to the line wire
. Looking at the end view, this 'Armature1 is placed
between the poles p p of the electro-magnet E M, and is there
rotated uniformly. When the wire is, in the position E, the
movement of its outer edge is a long the force lines, no cutting is
done, and up E.M.F. is produced; but when at D the lines are
cut at the highest speed, and the maximum E.M.F, exists in the
conductor. A careful examination will shew that the direction of
BJMLF. is changed twice in a revolution, and the diagram is